Body fluid samples such as blood, plasma, urine, interstitial fluid etc may be analyzed for various components or properties, such as glucose, cholesterol, pathogens, drugs of abuse, coagulation, hematocrit etc. For testing purposes, a body fluid sample is applied to a test element containing reactive ingredients. A reaction occurs and a result is produced.
Tests elements may be supplied in multiple numbers arranged along a carrier tape, in housings commonly referred to as cassettes. Test cassettes with two chambers, one containing unused test material and the other for storage of used test material are known for use in laboratory analysers, as well as portable analysers.
Test elements located on carrier tapes must be drawn out of a supply chamber to a reading station where a liquid sample is applied and analysed. Each test element is spaced apart from the adjacent element by sufficient distance to enable a test element to be at the reading station while the following element remains protected in the supply chamber.
It is possible to the draw the tape out of the supply chamber by motorised, mechanical or manual means. In all cases, it is necessary to ensure the test element is correctly positioned over the reading station before a sample is applied and analysed, in order to avoid inaccurate results or an error message.
In the case of manually driven tape cassettes, it may be possible for the user to simply watch the test element appear from the supply chamber and continue winding until it is over the reading station. In practice, the user may wind the test tape too far if winding quickly or if distracted during the winding. As the tape can only be wound one way this would mean the test is wasted and another test element would need to be used.
In the case of an automated system, the motor needs to be turned on and off at the correct moment to ensure the element is located over the reading station. To achieve this, the analyser device must utilise a positioning system. Such a system requires a sensing method. This may be optical, inductive, mechanical, capacitive, magnetoresistive or hall effect for example. In selecting a sensing option for a handheld battery operated medical device, cost, reliability, power consumption and resolution are extremely important factors to be considered.
The prior art contains examples of several positioning systems. U.S. Pat. No. 3,526,480 Findl et al., describes a carrier tape with sprocket holes to index positions. Multilayered porous elements impregnated with appropriate reagents are bonded along the carrier tape. Information is encoded on the carrier tape adjacent to each reaction site in the form of magnetic coding or punch holes. This information identifies the sample being analysed, the test being run and even the result of the analysis, but does not assist in the positioning. Creating sprocket holes is impractical with narrow, thin foil carrier tapes (similar in dimensions to audio tape) which are preferred in portable devices.
U.S. Pat. No. 4,954,319 Koizumi et al., describes a positioning system for a long film test cassette that has either colorimetric analytical test elements or electrode analytical test elements laminated to a carrier tape. A roller is pressed directly against the tape to measure the movement of the tape. To avoid tape slippage the roller perforates the tape with needle protrusions or has a rasp like surface that bites into the tape. The roller can become contaminated by sample if down line from the sample application site. Alternatively if the roller is situated before the sample application site it can damage the test elements.
U.S. Pat. No. 8,313,696 Seidenstricker et al., proposes a tape positioning system that uses optical sensing of a plurality of positioning marks that are printed on the carrier tape. Printing on the carrier tape is an additional manufacturing step, which increases the complexity of the manufacturing process and the cost. As cassettes are a consumable it is desirable to minimise their cost for the benefit of users and producers. In order to read the positioning marks, the carrier tape needs to be scanned by an optical reader. This has the disadvantage of adding to the cost of the device as well as it's power consumption. U.S. Pat. No. 8,282,893 B2 Schosnig et al. further develops this positioning system.